client: add a sentinel error for ParseFields

This error allows the caller to check whether a candidate struct has any
setec-tagged fields to decide whether to include it in a store config.

client/setec/fields.go

@@ -16,6 +16,11 @@ import (
 	"golang.org/x/exp/slices"
 )
 
+// ErrNoFields is a sentinel error reported by ParseFields when its argument is
+// a valid pointer to a struct, but does not contain any matching setec-tagged
+// fields. The caller should use [errors.Is] to check for this error.
+var ErrNoFields = errors.New("no setec-tagged fields")
+
 // ParseFields parses information about setec-tagged fields from v. The
 // concrete type of v must be a pointer to a struct value with at least one
 // tagged field.  The namePrefix, if non-empty, is joined to the front of each
@@ -28,7 +33,7 @@ func ParseFields(v any, namePrefix string) (*Fields, error) {
 		return nil, err
 	}
 	if len(fi) == 0 {
-		return nil, fmt.Errorf("type %v has no setec-tagged fields", reflect.TypeOf(v).Elem())
+		return nil, fmt.Errorf("type %v: %w", reflect.TypeOf(v).Elem(), ErrNoFields)
 	}
 	return &Fields{prefix: namePrefix, fields: fi}, nil
 }

client/setec/fields_test.go

@@ -140,8 +140,14 @@ func TestParseErrors(t *testing.T) {
 	}
 	t.Run("NonPointer", checkFail(struct{ X string }{}, "not a pointer"))
 	t.Run("NonStruct", checkFail(new(string), "not a pointer to a struct"))
-	t.Run("NoTaggedFields", checkFail(&struct{ X string }{}, "no setec-tagged fields"))
 	t.Run("InvalidType", checkFail(&struct {
 		X float64 `setec:"x"` // N.B. not marked with JSON
 	}{}, "unsupported type"))
+
+	t.Run("NoTaggedFields", func(t *testing.T) {
+		_, err := setec.ParseFields(&struct{ X string }{}, "")
+		if !errors.Is(err, setec.ErrNoFields) {
+			t.Fatalf("ParseFields: got %v, want %v", err, setec.ErrNoFields)
+		}
+	})
 }